Apparatus for measuring the energy and current of an accelerator electron beam including apertured incident and exit electrodes



Nov. 4, 1969 J. W..MOTZ 3,

' APPARATUS FOR MEASURING THE ENERGY AND CURRENT OF AN ACCELERATOR ELECTRON BEAM INCLUDING APERTURED INCIDENT AND EXIT ELECTRODES Filed Jan. 16, 1968 ELECTRON ELECTRON BEAM UTILIZATION ACCELERATOR MEANS Fzlg. 1

L I 20 23 T f I E 6 1ov .ov F I HOOOV z ZZMLLER MOTT-BQRN 29" 5 a INVENTOR O.) I l I BY SCATTERING ANGLE AGENT United States Patent [1.8. CI. 324--71 4 Claims ABSTRACT OF THE DISCLOSURE A metal foil and two apertured electrodes disposed on opposite sides of the foil are centered on the axis of an accelerator electron beam. The thickness of the foil, and the geometry and electric potentials of the electrodes, are arranged so that the electrodes selectively collect the secondary electrons emitted from the incident surface of the foil, and the elastically scattered electrons from the exit surface of the foil. The energy of the electron beam is then given by the ratio of the current due to the secondary electrons to the current due to the elastically scattered electrons; and the current of the electron beam is given by the current due to the secondary electrons.

BACKGROUND OF THE INVENTION This invention relates to electron accelerators, and more particularly to an apparatus for measuring the energy and current of accelerator electron beams.

In many applications of accelerator electron beams, it is desirable to monitor the energy and current of the beam, in order to control these properties of the beam, or the exposure time of the beam, or the like. Although the prior art contains many devices for measuring either the energy or current of an electron beam, it does not contain, so far as known, a simple device for measuring both the energy and current of the beam, independently of the accelerator. The object of this invention is to provide such a device.

SUMMARY OF THE INVENTION In the present invention, a metal toll is disposed across the path of the accelerator electron beam, and apertured electrodes for collecting the electrons scattered and emitted from the foil are disposed on opposite sides of the foil. The system is arranged so that the electrodes selectively collect the electrons that are related to the energy and current of the primary electron beam.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of an exemplary embodiment of the invention;

FIG. 2 and FIG. 3 are enlarged cross sectional views of portions of alternative exit electrodes;

FIG. 4 is an enlarged cross sectional view of a collimating electrode for use in the apparatus; and

FIG. 5 is a graph of the ratio of inelastically to elastically scattered electrons as a function of scattering angle.

FIG. 1 shows an electron accelerator arranged to project a beam of electrons 12, of energies greater than about 50 kev., along an axis 13 to an associated means 14 for using the electron beam. The apparatus 16- of this invention is disposed in the path of the beam 12 in order to measure the energy and current of the beam. The apparatus 16 includes a thin foil 18 of aluminum, gold, or other suitable metal, and two apertured electrodes 20 and 22 that are disposed on opposite sides of the foil. In the specific embodiment shown in FIG. 1,

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the foil 18 and electrodes 20, 22 are circular in cross section, and are mounted in a tubular dielectric housing 23. The foil 18 is supported by a ring 19 which is seated in a circular groove in the housing 23 arranged so that the foil lies in a plane perpendicular to the longitudinal axis of the housing. The incident electrode 20 (it is opposite the surface of the foil 18 on which the beam is incident) comprises an annular disc which also is seated in a groove in the housing 23, parallel to the foil 18. To fit the foil 18 and the electrode 20 in their grooves, the housing 23 may initially be split along its length, the elements placed in the grooves, and the housing then glued or otherwise secured together. The exit electrode 22 (opposite the beam exit surface of foil 18) comprises a hollow cylinder which is slidable in the housing 23, a screw 21 being provided to clamp the cylinder at the selected position.

In accordance with this invention, the foil 18 and exit electrode 22 are maintained at ground potential (see FIG. 1), and the incident electrode 20 is maintained at a positive potential with respect to ground by a DC potential source 24. A ratio microammeter 26 is connected between the exit electrode 22 and ground, and a microammeter 28 and the ratio microammeter 26 are connected between the negative terminal of the DC potential source 24 and ground. The microammeter 28 thus measures the current I; collected by the incident electrode 20, and the ratio microammeter 26 measures the ratio of the incident electrode current I, to the exit electrode current 1 The current I, in general consists of primary electrons backscattered from the foil 18, and secondary electrons ejected from the foil by the primary electrons. In this invention, the component of I due to backscattered electrons is suppressed by using a foil 18 that weighs less than about 200 ,ug./cm. The suppression effected by such a foil may readily be demonstrated, by adjusting the DC potential source 24 to about --1 kv. (this potential on the incident electrode 20 avoids the collection of any secondary electrons, which have energies very much less than 1 kev., but does not affect the collection of the backscattered electrons, which have energies very much greater than 1 kev.) and then decreasing the thickness of the foil 18: it will be found that the collected current (due solely to the backscattered electrons) essentially vanishes for thicknesses less than about 200 ng./cm. Accordingly, in this invention wherein the foil 18 is less than the above critical value, and the incident electrode 20 is operated with a positive potential from source 24, the current I, is produced only by the secondary electrons ejected from the foil by the primary electrons. In order to collect all of the secondary electrons, the potential of source 24 should be about 200-1500 v., and preferably about 300-1000 v.

The current I collected by the exit electrode 22 in general consists of elastically scattered electrons resulting from electron-atom collisions, inelastically scattered electrons resulting from electron-electron collisions, and secondary electrons ejected from the exit surface of the foil 18 by the primary electrons. In this invention, the contribution of the low energy secondary electrons is suppressed, because the electrode 22 is maintained at ground potential by the low impedance circuit through the ratio microammeter 26. Also the contribution of the inelastically scattered electrons is minimized, by adjusting the cylindrical exit electrode 22 Within the housing 23 so that the angle 0 between the axis of the housing and a line connecting the center of the foil 18 and the near edge of the electrode 22 is less than about 20. As shown in FIG. 5, the ratio of the inelastically scattered electrons to the elastically scattered electrons (as given by the ratio of Z times the Mller cross section per atom to the Mott-Born cross section per atom) is a minimum and substantially constant for scattering angles of less than about 20. The critical angle decreases with increasing electron energies, as illustrated by the family of curves in FIG. 5. Hence, by adjusting the position of electrode 22 to make less than about 20 (by an amount depending on the anticipated energy range of the beam), the current I collected by the electrode 22 is substantially proportional to the elastically scattered electrons.

The ratio microammeter 26 measures the ratio of the current I, to the current 1 From the prior art, it is known that the current 1, due to the secondary electrons ejected from the foil, is inversely proportional to the square of the electron velocity:

It is also known that the current 1,, produced mostly by elastically scattered electrons, is inversely proportional to the square of the electron velocity times momentum:

1 I a p Accordingly, the ratio of I to I as measured by the meter 26 is proportional to the square of the momentum, and hence proportional to the energy of the electron beam.

It is further known from the art that the current I, produced by the secondary electrons ejected from the incident surface of the foil 18 is proportional to the current of the primary electron beam 12.

From the foregoing, it will be seen that the energy of the electron beam 12 projected by the accelerator of FIG. 1 is given by the ratio microammeter 26, and that the current of the beam 12 is given by the microammeter 28. The proportionality constants relating these meter currents to the energy and current of the beam depend on the thickness, atomic number Z, and surface condition of the foil 18, and the angle 0 and the length of the exit electrode 22; and for a given foil and exit electrode geometry, these constants can readily be computed with an estimated accuracy of about ten percent. To obtain greater accuracy, the meters 26, 28 may be calibrated with, for example, a constant potential electron accelerator and a Faraday cup, by accurately measuring the accelerator potential and the current collected by the Faraday cup.

The apparatus of this invention thus provides a simultaneous measurement of the energy and current of an accelerator electron beam, with negligible interference of the beam, during continuous operation of the accelerator. In addition, the apparatus is simple and reliable, and operable over a wide range of electron energies from about 50 kev. to several mev.

To collect the elastically scattered electrons produced by accelerator beams of about 500 kev. and up, it may be desirable to arrange the exit electrode 22, shown in FIG. 1 as a cylinder, as a cup 30, FIG. 2, or as a well 32, FIG. 3. For low electron energies, the electrode 22 may comprise an annular disc (not shown) similar to the disc used for the incident electrode 20 in FIG. 1.

As mentioned above, it is usually desirable to maintain the incident electrode 20 at about 300-1000 v. with respect to the foil 18. At these potentials, some defocussing of the beam 12 may occur, which may readily be corrected if desired by positioning a collimating electrode 34, FIG. 4, in front of the incident electrode 20, and maintaining this electrode 34 at about 50 to 100 v. (e.g., v.) with respect to foil 18.

In a typical embodiment of this invention, the current I, is approximately one percent of the current of the primary electron beam 12, and the current ratio, li/le, may cover a wide range of values extending for example from 0.1 to 10. The microammeter 28 and the ratio microammeter 26 thus advantageously consists of transistorized electronic circuits which are capable of measuring a wide range of currents, ranging for example from 10"" to 10 amperes and which are capable of measuring either pulsed or DC. currents.

I claim:

1. An apparatus for measuring the energy and current of an accelerator electron beam, comprising a metal foil weighing less than about 200 ,ug./cm. disposed transversely to the axis of said beam,

an apertured incident electrode disposed opposite the surface of said foil on which said beam is incident, with the aperture thereof substantially centered on said beam axis,

means for maintaining said incident electrode at a positive potential with respect to said foil to collect substantially all of the secondary electrons ejected from said foil by said beam,

an apertured exit electrode disposed opposite the surface of said foil from which said beam exits, with the aperture thereof substantially centered on said beam axis,

means for maintaining said exit electrode at about the same potential as said foil to minimize the collection of secondary electrons,

said exit electrode being spaced from said foil so that the angle between said beam axis and a line connecting the intersection of said beam axis and foil with the adjacent outer edge of said exit electrode is less than about 20, to minimize the collection of inelastically scattered electrons, and

means for measuring the currents collected by said incident and exit electrodes.

2. An apparatus as set forth in claim 1, wherein said current measuring means includes a means for measuring the ratio of the current collected by said incident electrode to the current collected by said exit electrode, to indicate directly the energy of said electron beam.

3. An apparatus as set forth in claim 1, wherein an apertured collimating electrode is disposed in front of said incident electrode, and

means for maintaining said collimating electrode at a negative potential with respect to said foil, to reduce the defocussing of said electron beam by said incident electrode.

4. An apparatus as set forth in claim 3, wherein the potential of said incident electrode with respect to said foil is maintained at about 300 to 1000 volts, and the potential of said collimating electrode with respect to said foil is maintained at about -50 to volts.

References Cited UNITED STATES PATENTS 3,054,896 9/1962 Jones et al 250-419 3,098,998 7/ 1963 Smith.

EDWARD E. KUBASIEWICZ, Primary Examiner US. Cl. X.R. 25 049.5 

